The present invention relates to a toner, particularly a negatively chargeable toner, for developing electrostatic images in image forming methods, such as electrophotography, and electrostatic printing. The present invention also relates to an image forming method and a process cartridge using the toner.
Hitherto, a large number of electro-photographic processes have been known, as disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363; 4,071,361 and others. In these processes, an electric latent image is formed on a photosensitive member comprising a photoconductive material by various means, then the latent image is developed and visualized with a toner, and the resultant toner image is, after being transferred onto a transfer-receiving material, such as paper, as desired, fixed by heating, pressing, heating and pressing, etc., to obtain a copy or a print. In the case of including the step of transferring a toner image, a step of removing residual toner remaining on the photosensitive member is ordinarily also included.
Known developing methods for visualizing electrical latent images with a toner may include, e.g., the magnetic brush method described in U.S. Pat. No. 2,874,063, the cascade developing method disclosed in U.S. Pat. No. 2,618,552, the powder cloud method disclosed U.S. Pat. No. 2,221,776, and a method using an electroconductive magnetic toner disclosed in U.S. Pat. No. 3,909,258.
As for the step of fixing the toner image onto a sheet material such as paper which is the final step in the above process, various methods and apparatus have been developed, of which the most popular one is a heating and pressing fixation system using hot rollers.
In the heating and pressing system, a sheet carrying a toner image to be fixed (hereinafter called xe2x80x9cfixation sheetxe2x80x9d) is passed through hot rollers, while a surface of a hot roller having a releasability with the toner is caused to contact the toner image surface of the fixation sheet under pressure, to fix the toner image. In this method, as the hot roller surface and the toner image on the fixation sheet contact each other under pressure, a very good heat efficiency is attained for melt-fixing the toner image onto the fixation sheet to afford quick fixation.
Recently, in place of hot rollers, there has been commercialized a fixing apparatus comprising a heating member and a pressing member which is disposed opposite to the heating member and presses a recording medium (such as paper) to contact the heating member via a film.
On the other hand, in recent years, there have been also desired high-quality copy or print images in accordance with the use of digitalized copying machines and fine toner particles.
More specifically, it has been desired to obtain a photographic image accompanied by characters, so that the character images are clear while the photographic image is excellent in density gradation faithful to the original. Generally, in a copy of a photographic image accompanied by characters, if the line density is increased so as to provide clear character images, not only the density gradation characteristic of the photograph image is impaired, but also the halftone part thereof is roughened.
Further, resolution failure (collapse) of line images and scattering are liable to be caused at the time of fixation as described above, so that the image qualities of the resultant copy images are rather liable to be deteriorated.
Further, in case where the line image density is increased, because of an increased toner coverage, a thick toner image is pushed against a photosensitive member to be attached to the photosensitive member in the toner transfer step, so that a so-called transfer failure (or a hollow image), i.e., a partial lack toner image (line images in this case), in the transferred image, is liable to be caused, thereby providing poor quality of copy images. On the other hand, in case where the gradation characteristic of a photographic image is intended to be improved, the density of characters or line images is liable to be lowered, thus providing unclear images.
In recent years, there has been obtained some improvement in density gradation characteristic by a system including image density readout and digital conversion. However, a further improvement has been desired.
Regarding density gradation characteristic, it is impossible to obtain a linear relationship between a developing potential (difference between a photosensitive member potential and a developer-carrying member potential) and a resultant (copy) image density. In a halftone region, a slight change in developing potential leads to a remarkable change in image density. This provides a difficulty in obtaining a satisfactory density gradation characteristic.
Generally, copied images appear clearer because of an edge effect of attracting an increased amount of toner so that clear line images can be retained in the case where a maximum density of ca. 1.30 is attained at a solid image part which is less affected by the edge effect.
In case of a photographic image, however, the maximum density of a photograph appears less at a glance because of its surface gloss but actually amounts to a very high image density level of 1.90-2.00. Accordingly, in a copy of a photographic image, even if the surface gloss is suppressed, a solid part image density of ca. 1.4-1.5 is required since a density increase due to the edge effect cannot be expected because of a large image area.
Accordingly, in providing a copy of a photographic image accompanied by characters, it becomes very important to obtain a developing potential-image density relationship which is close to the first order (linear) one and also a maximum image density of 1.4-1.5.
Further, the density gradation characteristic is liable to be remarkably affected by the saturation charge and the charging speed of a developer used. In case where the saturation charge is appropriate for the developing conditions, a developer showing a slow charging speed provides a low maximum image density, thus generally thin and blurred images in the initial stage of copying. In this case, however, satisfactory images can be obtained if the maximum image density is ca. 1.3, as described above, thus being able to obviate an adverse effect of the slow chargeability. Even in case of slow charging speed, the initial copy image density is increased if the saturation charge is increased. However, on continuation of copying, the charge of the developer is gradually increased to finally exceed an appropriate charge for development, thereby resulting in a lower copy image density. Also in this case, no problem occurs in line images if the maximum image density is ca. 1.3.
From the above, it is understood that a photographic image is more remarkably affected by the saturation charge and the charging speed of a developer than a line image.
In case where a smaller particle size toner is used, the dispersion state of a charge control agent and a colorant remarkably affects the chargeability of the toner.
A toner for developing electrostatic images may generally contain a dye called a charge control agent for controlling the chargeability of the toner. In order to provide a toner with a negative chargeability, chromium complex compounds have been principally used.
Japanese Laid-Open Patent Application (JP-A) 60-170864, describes that, among such chromiun complex compounds, those having a good mutual solubility with a binder resin show a uniform negative chargeability and provide clear copy images but are liable to be accompanied with difficulties, such as forming a toner residue on a photosensitive member due to cleaning failure and filming. Those chromium complex compounds being insoluble with a binder resin (particularly in a polyester resin) show good chargeability and also good anti-filming characteristic.
However, a metal complex salt compound insoluble or incompatible with a binder resin shows a poor dispersibility. Accordingly, when a toner containing such a metal complex salt compound is formulated into fine particles, the toner is liable to be charged excessively particularly in a low-humidity environment, thus leading to fog or a reduction in density. This is because a fine particle size fraction and a coarse particle size fraction formed through a pulverization step of toner production are caused to have remarkably different contents (weight ratios) of the charge control agent (i.e.,so-called localization of a charge control agent), so that toner particles are caused to have different chargeabilities.
In case where a fine powder fraction and a coarse powder fraction recovered from the classifying step are re-utilized as a material for toner production, the above-mentioned liability of localization of a charge control agent is further promoted to cause difficulties, such as a lowering in image density and fog due to a toner electrification insufficiency under a low-humidity condition. For this reason, it has been hitherto difficult to reutilize both the fine powder and coarse powder by-produced in the classification step for toner production, and coarse powder alone has been reutilized as proposed in JP-A 3-209266. JP-A 61-155464 and JP-A 62-177561 have proposed an azo-type iron complex as a charge control agent showing good dispersibility within a binder resin. A toner containing the azo-type iron complex is, however, accompanied with difficulties, such as a slow rate of electrification and a lowering in image density after a long period of standing or in a high humidity environment. In recent years, a smaller particle size (at most 9 xcexcm in terms of a weight-average particle size (diameter)) is recommended for providing high-quality images. A small particle size toner is liable to have a remarkably high charge under a low-humidity condition and cause difficulties, such as thinning of line images, a lowering in image density and occurrence of reversal potential fog caused by a toner charged to an opposite polarity due to charging failure on a developer-carrying member, such as a developing sleeve, due to the copresence of the excessively charged toner.
In order to improve the chargeability of a toner containing such an azo-type iron complex, JP-A 1-306862 has proposed a silicone resin-coated carrier which has a high chargeability-imparting effect, and JP-A 2-153362 has proposed a developing apparatus including an improved toner layer thickness-regulating member and an improved toner replenishment-assisting member. In these proposals, the developing performance of the toner is retained by charge-imparting or -assisting members and it is difficult to retain good image quality for a long period due to deterioration or soiling of the charge-imparting or -assisting member.
An object of the present invention is to provide a toner for developing electrostatic images having solved the above-mentioned problems and capable of retaining a high-quality image forming performance for a long period.
An object of the present invention is to provide a toner having a good dispersibility of a charge control agent and a uniform chargeability, capable of retaining a high image density for a long period and capable of providing images free from fog and with a high resolution.
Another object of the present invention is to provide a toner which can be quickly charged and can provide good toner images similarly as before standing even after storage for a long period or in a high-humidity environment.
Another object of the present invention is to provide a toner which can provide high-quality images without using a charge-assisting member.
Another object of the present invention is to provide a fine particle size toner which can provide satisfactory developed images for a long period under various environmental conditions even in case of providing high-resolution developed images.
Another object of the present invention is to provide a toner which allows re-utilization of fine powder and coarse powder by-produced in the classification step in toner production.
Another object of the present invention is to provide a toner highly suitably adapted to an electrophotographic process not adversely affecting a photosensitive member or a developer-carrying member.
A further object of the present invention is to provide an image forming method and a process cartridge using such a toner as described above.
According to the present invention, there is provided a toner for developing electrostatic images, comprising:
(a) a binder resin,
(b) a long-chain alkyl compound represented by the following formula (1), (2) or (3):
CH3"Parenopenst"CH2"Parenclosest"xCH2OHxe2x80x83xe2x80x83(1),
wherein x denotes an average value in the range of 35-150, 
wherein x denotes an average value in the range of 35-150; z denotes an average value in the range of 1-5, and R denotes H or an alkyl group having 1-10 carbon atoms,
CH3"Parenopenst"CH2"Parenclosest"yCH2COOHxe2x80x83xe2x80x83(3),
wherein y denotes an average value in the range of 35-15; and
(c) an azo-type iron complex compound represented by the following formula (4); 
wherein X1 and X2 independently denote hydrogen atom, lower alkyl group, lower alkoxy group, nitro group or halogen atom; m and mxe2x80x2 denote an integer of 1-3; R1 and R3 independently denote hydrogen atom, C1-8 alkyl or alkenyl, sulfonamide, mesyl, sulfonic acid group, carboxy ester group, hydroxy, C1-8 alkoxy, acetylamino, benzoylamino or halogen atom; n and nxe2x80x2 denote an integer of 1-3; R2 and R4 denote hydrogen atom or nitro group; and A+ denotes a cation including 75-98 mol. % of ammonium ion and another ion selected from the-group consisting of hydrogen ion, sodium ion, potassium iron and mixtures thereof.
According to another aspect of the present invention, there is provided an image forming method, comprising:
a charging step of supplying a voltage to a charging means in contact with a member to charge the member,
a step of forming an electrostatic image on the charged member,
a developing step of developing the electrostatic image with a toner as described above to form a toner image on the member,
a transfer step of transferring the toner image to a transfer-receiving material directly or via an intermediate transfer member, and
a fixing step of fixing the toner image onto the transfer-receiving material.
According to a further aspect of the present invention, there is provided a process-cartridge, comprising at least a developing means and a photosensitive member,
the developing means and the photosensitive member being integrated into a cartridge which is detachably mountable to a main body of an image forming apparatus,
wherein the developing means contains a toner as described above.